1 percent of all children have congenital cardiac disease at birth. Some of these have pulmonary vascular disease. Correct assessment of the pulmonary vascular obstructive disease is necessary to permit correct judgement regarding operative repair. Now, pulmonary vascular resistance is used to estimate the degree of pulmonary vascular obstructive disease, but it is far from satisfactory. The specific objectives of this research are (1) to measure pulmonary vascular impedance in children with heart disease and (2) to determine if this measurement is superior to pulmonary vascular resistance for assessment of the presence and degree of pulmonary vascular obstructive disease. Accordingly, it is necessary to calibrate pulmonary vascular obstructive disease, and it is proposed that the ratio of media to diameter of all pulmonary vessels between 50-150 microns be measured according to the technique of Wagenvoort. Pulmonary vascular impedance computation requires instantaneous measurement of main pulmonary artery pressure and flow. Pressure will be measured with a catheter tip manometer. Instantaneous blood flow is more difficult to measure, and at present an electromagnetic probe or Wormersley analysis is thought to give the best results. If newer techniques are proved more effective during the course of the study, they will be used. Harmonics and basic frequency will be determined by submitting the tape recorded instantaneous pressure and flow to a computer of average transients which will have the capability of continuously outputting the signal to a spectral analyzer. The spectral analyzer will analyze the two waveforms for harmonic amplitude and phase angle. The modulus (pressure at a given harmonic divided by the flow at that harmonic) will be computed for each basic frequency and harmonic. The distance to the point of reflection can be computed after the wave velocity has been measured by means of two catheter spaced manometers. Additionally, the reflection coefficient can be computed. Conventional pulmonary vascular resistance will be computed. The various computed information will be compared to the ratio of media to arterial wall thickness to determine if the impedance technique can predict the degree of pulmonary vascular obstructive disease found by pathological examination.